This invention relates to an engine control system and, more particularly, to an engine control unit for controlling the spark timing of an engine.
To achieve fuel economy, reduced unwanted exhaust emissions and improved engine performance and drivability, it is desirable to accurately control the various operations of an internal combustion engine such as spark timing, fuel metering and idle speed. Because of improved accuracy and lower costs, digital engine controllers have been proposed. The engine controllers employ a digital computer which reads an engine load sensor at intervals for use in calculating appropriate values for fuel-injection pulse width and ignition-system spark timing from different programmed relationships. It is the conventional practice to calculate an appropriate value for fuel-injection pulse width as a function of an engine load value read just before the intake phase of engine operation and calculate an appropriate value for ignition-system spark timing as a function of another engine load value read near the end of the compression phase of engine operation. However, the engine load will vary to an great extent, particularly during rapid acceleration, between the time when the engine load sensor is read for calculation of the fuel-injection pulse width and the time when the load sensor is read for calculation of ignition-system spark timing. Under this condition, the resulting spark timing will be different from an optimum timing for the amount of fuel supplied to the cylinder, causing misfire or other undesired engine operation.